ftrace: prevent recursion

Impact: prevent unnecessary stack recursion

if the resched flag was set before we entered, then don't reschedule.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>